This invention relates to an apparatus for data transfer with a rotating data storage disk such as a flexible magnetic disk having a multiplicity of annular tracks arranged concentrically thereon. More specifically, the invention deals with a system in such apparatus for automatically positioning a data transducer on a reference track, usually the outmost Track Zero, on the disk when the apparatus is electrically powered on.
In the rotating flexible magnetic disk apparatus, or disk drive according to more common parlance, the transducer must be recalibrated on Track Zero, or outermost valid track, on the disk when the apparatus is electrically turned on. Two schemes have been known and practiced for such recalibration. One is to incorporate a "power on" sensor circuit in the disk drive, causing the same to recalibrate the transducer independently of a host system with which the disk drive is interfaced. The other is to cause the disk drive to recalibrate the transducer under the direction of the host system.
The first scheme requires the disk drive to incorporate a recalibration circuit in addition to the "power on" sensor circuit. Consequently, when the complete data processing system, comprising the host and the disk drive or drives, is switched on, power is consumed immediately for recalibration purposes in addition to that for other working parts of the system. The total power requirement of the data processing system must therefore be rather inconveniently large.
The above inconvenience is absent from the second mentioned scheme because transducer recalibration under the control of the host takes place shortly after, rather than immediately when, the system is powered on. This second scheme has its own weakness, however. Disk drives are slave units which may be connected to hosts of various kinds made by various manufacturers. Some hosts are well calculated so that the transducer may be infallibly positioned on Track Zero after the system is switched on, no matter whatever position on the disk the transducer may have been in at that time.
There are, however, other, less sophisticated hosts that cause the stepper motor, which is included in the disk drive for track seek operation of the transducer, to be energized only in such a direction that the transducer travels radially outwardly of the disk when the system is switched on. The radially outward travel of the transducer is terminated when the disk drive signals the host that the transducer has arrived at Track Zero. A problem with this second known type of recalibration method is that the optical Track Zero sensor which is customarily built into the disk drive is inherently susceptible to sensing errors, possibly indicating that the transducer is on Track Zero when it is actually one or two tracks away in either direction from the reference track.
The problem will be better understood by considering the transducer recalibration system disclosed in U.S. Pat. No. 4,594,620 to Shoji et al. This prior art system determines that the transducer is on Track Zero when the Track Zero sensor puts out a signal to that effect, when the host supplies a signal commanding the radially outward travel of the transducer, and, at the same time, when the stepper motor has its predetermined sets of windings energized in a predetermined direction to position the transducer on Track Zero. The prior art system has proved unsatisfactory because the predetermined windings of the stepper motor are not necessarily energized in the predetermined direction only when the transducer is on Track Zero and because, as aforesaid, the Track Zero sensor is incapable of accurately sensing that the transducer is on the reference track.
Thus, according to the prior art, the host has been prone to be erroneously informed that the transducer is on Track Zero when it actually is not. Subsequent track seek operations of the transducer will of course be totally unsuccessful if it is not correctly recalibrated when the system is switched on.